In the traffic planning and highway management industry, the precise speed and distance of a vehicle is often required while the vehicle is being driven. In order to measure these parameters, a transmission sensor is tapped into the vehicle speed sensor (VSS) signal, and the transmission sensor sends the data to a distance measuring instrument (DMI), which displays and/or records into memory, the precise vehicle speed and distance travelled. An example in-vehicle commercially available DMI is a RAC Plus™ series instrument manufactured by JAMAR Technologies, Inc.™.
Every year, many DMI units are sold and each requires a sensor kit to be installed in each vehicle in which the DMI is to be used. The sensor kit provides an interface to tap into the VSS signal, which is generated in all vehicles and used in various sub-systems in the vehicle, including the speedometer. The location and colour coding for the cable that carries the VSS signal can vary for each make and model of vehicle. As a result, customers typically are required to specify in what vehicle(s) they are planning to use the DMI, in order to enable the vendor to look up the location of the cable and choose the appropriate sensor kit and instructions to enable the customer to install the sensor. The actual installation of the sensor kit is then required, for which the customer is responsible. Often the location of the cable carrying the VSS signal is under the hood, in the dash, or in an otherwise hard to reach area of the vehicle including underneath the vehicle and along the transmission. Once the sensor has been installed and a separate power cable is connected, the DMI can be plugged into the cable running from the sensor and used by the customer.
The VSS signal comprises a sequence of pulses that are generated by a VSS module. Each pulse equals a fixed distance that the vehicle has travelled. The sensor obtains the VSS signal, amplifies it, and then sends the amplified signal to the DMI as a series of pulses. The DMI may then count the pulses coming from the sensor and from that data, calculates the distance travelled. For example, if every pulse represents 1,000 feet, then 1000 pulses would equal 1000 feet. However, typically, the pulses do not necessarily represent a known distance and thus the DMI needs to be calibrated in such cases after installing the transmission sensor. This involves driving a known distance (e.g. 1000 feet) in a calibration mode of the DMI. The DMI then counts the number of pulses and then calculates a calibration factor that is used to convert from pulses to distance thereafter.
There are several difficulties and drawbacks associated with existing sensor kits used with the DMI units. For instance, the sensor kit can be difficult to install. On some vehicles, a lift may be required to access the cable carrying the VSS signal. Also, in most cases, the wire that taps into the VSS-wire needs to be routed through the firewall of the vehicle so that it can be accessed from within the cab of the vehicle where the DMI is typically located and the wiring can be prone to corrosion if under the hood. This also often requires drilling holes through the firewall, which adds to the time and complexity of the installation process. Moreover, if the installation of the transmission sensor is done incorrectly, this can interfere with the operation of the ABS braking systems in a vehicle, which in turn can create a dangerous hazard.
The VSS signal is also a low voltage signal. As such, the wire that runs from the ‘tap-in’ point into the cab is susceptible to electromagnetic interference (EMI) if the wiring is not installed carefully. This noise can affect the accuracy of the data in subtle ways. In some cases, the user may not ascertain that a problem exists until many measurements over a great deal of time have been taken. This can then put previous recordings into doubt. Also, in order to even use the DMI, the desired vehicle that is to be used must have the sensor kit already installed. If it is not, the DMI cannot be used in that vehicle or others.
Given that newer automotive systems are becoming more and more complex, it is becoming more and more difficult to find a specific cable that carries only the VSS signal. In new automotive systems, fewer wires are used, sub-modules in the vehicle communicate with each other over buses, and signals are multiplexed akin to computers in a network. This makes it harder to tap the system for a specific signal. Current DMI units cannot be used with such automotive systems since the transmission sensor is unable to consistently tap into the desired signal. Additionally, the sensor and the DMI unit require a separate power feed (often coming from the battery) and the sensor must be calibrated before being used with the DMI. If changes are to be made to the vehicle (such as changing the tires), then the sensor must also be re-calibrated.
These requirements make the DMI units compatible with only those vehicles having a sensor installed therein and make them susceptible to changes in the operation of the vehicle and the vehicle type.